This has been a hobby of mine for several decades now. Let me share some of my favorite common signs of intelligence I have observed.
Talk only to share information
To them, small talk is simply that. Small. Meaningless even. It’s a waste of time.
They proactively listen to others and digest what was said before explaining their counter-argument.
They bounce ideas and hypotheses off of others rather than discuss random pop psychology or the latest news on a celebrity couple.
In short, they shamelessly focus on what’s important.